This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2001-053495, filed Feb. 28, 2001, the entire contents of this application are incorporated herein by reference.
This invention relates to an improvement of thermal properties of poly(tetrafluoroethylene) resin feeds by ionizing radiations. More particularly, the invention relates to a process for producing radiation-modified poly(tetrafluoroethylene) resin feeds by applying ionizing radiations to unsintered poly(tetrafluoroethylene) resins under mild conditions so that only their melting temperature is shifted to the lower end without changing the quantities of heat of fusion and crystallization. The thus modified poly(tetrafluoroethylene) resin feeds are characterized in that their moldings have the same melting temperature, heat of fusion, crystallization temperature and heat of crystallization as the moldings of the unirradiated poly(tetrafluoroethylene) resin feeds. The invention also relates to the radiation-modified poly(tetrafluoroethylene) resin feeds produced by the process.
Poly(tetrafluoroethylene) resins are engineering plastics having high resistance to heat, chemicals and friction, high water repellency, good lubricating property and high electrical insulation. Because of these features, poly(tetrafluoroethylene) resins have heretofore been used in various industrial and consumer applications including electric wire coatings, tubes, pipes, packings, gaskets, linings, insulation tape, bearings and roofing membranes for air domes.
However, unlike the other polymeric materials, poly(tetrafluoroethylene) resins are seldom molded into various shapes on their own. For the very reason that they have high heat and chemical resistance, poly(tetrafluoroethylene) resins are difficult to shape by heating or dissolving in solvents.
With a view to improving the moldability of poly(tetrafluoroethylene) resins, it has been attempted to modify them by partial replacement of the tetrafluoroethylene molecules with a copolymerizable component. However, the introduced copolymerizable component not only causes substantial changes in thermal properties, such as the quantities of heat of fusion and crystallization, of the poly(tetrafluoroethylene) resins but also impairs their purity, thus causing deterioration in various characteristics of their moldings.
Of the various polymeric materials known today, poly(tetrafluoroethylene) resins are the easiest to deteriorate upon radiation exposure and the mechanical characteristics of their moldings decrease with absorbed dose whereas the quantities of heat of fusion and crystallization increase as more molecular chains are severed. Needless to say, poly(tetrafluoroethylene) resins in bulk powder form have substantially the same sensitivity to radiation as when they are shaped into moldings; following the exposure to radiation, the quantities of heat of fusion and crystallization increase as more molecular chains are severed and the mechanical characteristics of shapes molded from the irradiated poly(tetrafluoroethylene) resins are also inferior to those of the unirradiated resins. These effects of radiations are currently used to prepare a wax of poly(tetrafluoroethylene) resins by decomposing them.
It has recently been shown that poly(tetrafluoroethylene) resins are crosslinked by applying ionizing radiation in a special environment, i.e. at elevated temperature in the absence of oxygen. The crosslinked resins have higher heat resistance but, on the other hand, their moldability is impaired.
It is, therefore, the principal object of the invention to improve the moldability of poly(tetrafluoroethylene) resins by the simple means of applying radiation such that their purity is retained and that only their melting temperature is shifted toward the lower end without changing the quantities of heat of fusion and crystallization from those of unsintered poly(tetrafluoroethylene) resins.
More specifically, the invention provides radiation-modified poly(tetrafluoroethylene) resins which are characterized in that their moldings have the same melting temperature, heat of fusion, crystallization temperature and heat of crystallization as the moldings of the unirradiated poly(tetrafluoroethylene) resin feeds. The invention also provides a process for producing such radiation-modified poly(tetrafluoroethylene) resin.
In order to attain the stated object, the present inventors applied ionizing radiations to unsintered poly(tetrafluoroethylene) resins in bulk powder form under various conditions and made a close study of the relationship between the exposure conditions and each of the thermal properties, such as melting temperature, heat of fusion, crystallization temperature and heat of crystallization, of the resins both before and after shaping. As a result of their intensive studies, the inventors found the following new fact: when unsintered poly(tetrafluoroethylene) resin feeds were exposed to ionizing radiation in an absorbed dose of no more than 1,000 Gy at room temperature in air, only their melting temperature was shifted toward the lower end without changing the quantities of their heat of fusion and crystallization, whereby their moldability was improved; in addition, the moldings of the irradiated resin feeds had identical values of thermal properties, such as melting temperature, heat of fusion, crystallization temperature and heat of crystallization, to those of the moldings prepared from the unirradiated resin feeds. The present invention has been accomplished on the basis of this finding.
The present inventors previously invented a method in which poly(tetrafluoroethylene) resins were exposed to ionizing radiation, thereby producing radiation-modified poly(tetrafluoroethylene) resins having increased tear strength and an application for obtaining patent on this invention was already filed (Japanese Patent Application No. 2000-155902). The present invention provides a very effective method of using radiations in that the moldability of poly(tetrafluoroethylene) resin feeds could be improved by only shifting the melting temperature of unsintered resin feeds toward the lower end without affecting the thermal properties of the moldings to be prepared.